Fly Agaric - Amanita Muscaria
Probably the most easily recognised mushroom in UK and the basis for many illustrations for children's fairy stories etc. It really is this strikingly red in real life but it fades with age and becomes less attractive. I found many in the woods but this was the prize specimen and most of the others were old, faded and tattered.
They are widespread and quite common in the right habitat which is acidic soils and especially where there are birch trees. If you aren't very good at trees (and I'm not) then you may at least be able to identify Silver Birch by its silver bark so look around those. If you are lucky enough to find some then hunt around for that prime specimen.
After the initial dome shape they become theclassic convex mushroom shape and then flat topped, all the time getting more faded and ragged (See below).
OH , AND BY THE WAY, THEY ARE POISONOUS - SO BEWARE!